This invention relates generally to new and improved perforated composite panels, commonly called perforated face sheets, and to the method of molding them. More particularly, it relates to continuous fiber reinforced perforated composite panels and to the method of molding them.
Perforated panels of the type to which the invention pertains commonly are used for acoustic abatement in the gas turbine engine environment. In the past, these perforated panels have been made of aluminum because it is light and easily formed. However, aluminum has a weight in excess of what would be necessary using higher strength materials. Also, aluminum forms surface oxides which are difficult to bond to. Removal of the surface oxide through etching processes is costly and is very process-dependent.
Various efforts have been made to find a suitable substitute for the aluminum perforated face sheets, but to date none of the efforts have been entirely successful. One such effort consisted of a honeycomb core sandwich having one or more honeycomb cores. One such honeycomb core sandwich is described in U.S. Pat. No. 4,271,219.
In addition to the honeycomb core sandwiches, continuous fiber reinforced perforated composite panels have been fabricated to replace the aluminum face sheets. These perforated face sheets or composite panels have been fabricated using a thermoset matrix resin system. One of the major problems in fabricating these composite panels is the development of an efficient process for producing them. Generally, they have been formed over sacrificial plastic pins. Once the composite perforated face sheets have been consolidated over the sacrificial pins, the pins are broken and the composite is removed. Fabricating the composite panels in this fashion is expensive due to the multiple step processes necessary to form them. Also, the process does not always provide good uniformity of the face sheet hole penetrations. This is caused primarily to die lock of the panel on the sacrificial pins. Further still, the holes are non-uniform due to the fact that the matrix resin's viscosity lowers dramatically during the cure cycle, causing the area immediately around the sacrificial pin dies to become resin rich with resulting lack of continuous fibers in the localized area.
Another current method of fabricating perforated composite panels is to use a laser to cut individual holes through the solid laminate. This technique is likewise expensive since it takes approximately one and one-half seconds to cut each individual hole in the composite panel. Since there can be up to 25,000 individual holes in a panel, this technique is very inefficient. In addition, using a laser to cut the holes in a thermoset matrix fiber reinforced composite panel can cause substantial damage to both the matrix and the fiber in the localized area. These damaged areas are susceptible to moisture ingressions and crack propagation in long term fatigue conditions.
In another similar process, tows of composite materials are placed around the pins. This process also is expensive and still does not yield good fiber placement around the hole periphery.